The present invention is applicable to virtually any photolithographic process. The inventive process is particular applicable to the fabrication of lead frames which provide the metal leads extending from a packaged integrated circuit.
FIG. 1 is a simplified plan view of a lead frame strip 10, having a typical length on the order of 9 inches and a width on the order of about 2 inches. Strip 10 may, however, be any width depending on the required size of a lead frame. Strip 10 is formed of copper or other metal and is generally 1-20 mils thick, depending upon the required ruggedness of the resulting leads of the integrated circuit package. Strip 10 has a number of repeated lead frame 12 patterns which are formed by either stamping or a photolithographic process. Each lead frame 12 pattern must be precisely formed and precisely aligned with respect to the other lead frame 12 patterns on strip 10 and with respect to the strip 10 itself in order for an automated bonding machine to centrally position an integrated circuit chip on each lead frame 12 and bond the pads of the integrated circuit chip to the leads 13 of the lead frame 12. After the chips are bonded to the lead frames 12, the individual lead frames 12 are separated from the strip 10, and the chips are encapsulated.
For high lead-count integrated circuits, a width of each of leads 13 may be only on the order of a few mils (e.g., 9 mils). A typical customer specification for the lead frames 12 requires each lead frame 12 to be formed within a tolerance of 1 mil and the alignment of the lead frame 12 patterns with respect to one another and with respect to strip 10 to be also within 1 mil to ensure reliability in the automated bonding process.
FIGS. 2A and 2B illustrate one existing method, developed by National Semiconductor Corporation, for forming the lead frame strips 10 using a photolithographic process.
A blank copper web 14 is generally supplied on a reel, where the copper web 14 may have a length around 500 feet. A photoresist layer is laminated onto the copper web 14.
A proximity type mask 16 is provided with a first chrome pattern 18 corresponding to one or more identical lead frame 12 patterns. Since a conventional mask 16 size is about 12 inches by 6 inches, the image formed by the first chrome pattern 18 (taking up almost one half the length of mask 16) exposes about half the 9 inch strip 10. Dashed lines 20 on copper web 14 identify where the web 14 will be eventually cut to form separate strips 10. Two exposure steps of pattern 18, requiring a shift of copper web 14, are thus required to expose the entire 9 inch copper strip 10 to pattern 18.
Due to the various mechanisms used to shift and align the copper web 14 under the mask 16, fine airborne particles of copper and other contaminants may alight upon the chrome pattern 18, causing repeated defects in the resulting lead frame 12 patterns. FIG. 2A illustrates a particle of dirt 22 residing at an edge of pattern 18. A second particle 23 is shown residing near the center of pattern 18. When the photoresist on copper web 14 is selectively exposed to ultraviolet radiation 24 through pattern 18, repeated defects 25 and 26 in the exposed photoresist pattern 27 are formed by particles 22 and 23, respectively. For simplicity of illustration, it will be assumed that the pattern 18 is a single, solid rectangle, when in actuality pattern 18 would contain a number of lead frame 12 patterns.
In an attempt to avoid such repeated defects in the lead frame 12 patterns, a second chrome pattern 28 on mask 16 is provided which is identical to pattern 18 but has feature dimensions reduced by 2 mils. The reduction in the feature dimensions by 2 mils takes into account a positional tolerance of the copper web 14 of approximately 1.5 mils when the copper web 14 is shifted in the X direction 30 to cause the exposed photoresist pattern 27 to be aligned directly under pattern 28, as shown in FIG. 2B.
The previously exposed photoresist pattern 27 is now partially double-exposed to ultraviolet radiation 24 through chrome pattern 28. Since the overlap of the pattern 28 image and the pattern 18 image leaves a border of 2 mils, the edge defect 25 remains but the centrally located defect 26 is eliminated by the exposure overlap. If mask 16 were used to form lead frames 12, edge defect 25 would represent a 2 mil defect in the edge of a resulting lead 13 as shown in FIG. 3.
The exposed photoresist is then developed, and the copper web 14 is subjected to an acid etch to remove the portions of the copper web 14 not covered by the photoresist. Assuming mask 16 contains lead frame 12 patterns, each lead frame 12 pattern in strip 10 of FIG. 1 would contain a faulty lead 13 (FIG. 3) having a pit 32 or notch 2 mils wide in a lead 13 which may be less than 9 mils wide. Thus, the resulting strip 10 would not meet the customer specification and would have to be discarded. It is Applicants' experience that such rejections occur approximately 5% of the time.
Additionally, due to the 1.5 mil positional accuracy of the copper web 14, the required repositioning of the copper web 14 to position the first photoresist pattern 27 under the second pattern 28 for each strip 10 causes the alignment of the lead frame 12 patterns in strip 10 to exceed the customer's typical tolerance of 1 mil approximately 5% of the time. This results in another 5% of strips 10 discarded, totalling a loss of 10% on the average.
What is needed is a more reliable photolithographic method for producing patterns using a mask, and particularly for producing lead frame patterns.